While B1, B2 and B3 are all assumed to predict Y, also B2 is assumed to predict B3.

As I have never seen before neither such a level-2 mediation (b2 --> b3) nor an application of multilevel SEM for repeated cross-sectional surveys, I wonder if the model above would be feasible at all?

Are you thinking that the 15 surveys (years) have characteristics that make the sample respondents of a given year be more like each other than the respondents in the other years? If not, there would be no intraclass correlation and therefore no need for twolevel modeling.

The example above is still hypothetical. But according to some, considering individuals nested in repeated surveys as described might be considered to form a hierachical structure. If this is correct and under the assumption that there would be sufficient ICC in the dependent variable, would the model described in my first question be feasible? Best, Tom